The present invention relates to a method and apparatus for manufacturing metal pipe and more particularly to a forming method and apparatus for manufacture of electrically welded pipe.
Heretofore, for forming pipe or tube from a flat strip or a skelp, the following processes have been carried out.
(1) For rough forming, a skelp is fed into a pass formed by upper and lower rolls having a convex curved surface and concave curved surface respectively, and the skelp is thereby formed into a shallow U-shaped cross section by the engagement of upper and lower surfaces by the rolls.
(2) For intermediate forming, the U-shaped skelp is acted on by vertical rolls having concave curved surfaces designed to contact the entire outer surface of the skelp to bend the shallow U-shaped cross section skelp into a deep U-shaped cross-section skelp.
(3) For finish forming, pressure is applied in the width direction of the skelp by means of fin rolls designed to contact almost the entire outer surface of the skelp and the end portion of the skelp is held.
(4) Driving rolls drive the rough forming, intermediate forming and finish forming rolls respectively.
In the foregoing conventional processes, there exist the following problems.
(1) Since the skelp edge is threaded into the forming mill without the leading end being held, the cross section in the width direction of the skelp does not remain symmetrical with respect to the centerline of the pass due to non-uniformity or error in roll setting precision, etc., and biting into the fin pass stand is not carried out smoothly, thereby making the threading tremendously difficult and reducing the working efficiency and also making the operation impossible without a high degree of skill. The range of interchangeability of roll sizes in the rough forming and intermediate forming passes is extremely narrow, and separate sets of rolls are usually required to be provided for each diameter of pipe to be manufactured, and as a result, the roll cost is high.
(3) Roll change-over required for a change of diameter of the pipe to be manufactured takes a long time reducing the operating ratio of the apparatus and causing the productivity to be extremely low.
(4) Since the rolls and the skelp are in full contact with one another over the entire outside surface of the skelp, slipping occurs due to differences in peripheral speeds of the rolls and the skelp, thereby causing tremendous energy loss during the forming.
The present invention seeks to solve the foregoing problems.